Electronically operated remote control systems, such as garage door opener systems, home security systems, home lighting systems, gate controllers, etc., typically employ a portable, hand-held transmitter (i.e., an original transmitter) to transmit a control signal to a receiver located at the remote control system. For example, a garage door opener system typically includes a receiver located within a home owner's garage and coupled to the garage door opener. A user presses a button on the original transmitter to transmit a radio frequency signal to the receiver to activate the garage door opener to open and close a garage door. Accordingly, the receiver is tuned to the frequency of its associated original transmitter and demodulates a predetermined code programmed into both the original transmitter and the receiver for operating the garage door. To enhance security of wireless control systems, such as a garage door opener system, manufacturers commonly use encryption technology to encrypt the radio frequency signal sent from a transmitter to a receiver. One example of such an encryption method is a rolling code system, wherein each digital message sent from the transmitter to the receiver has a different code from the previous digital message.
As an alternative to a portable, hand-held original transmitter, a trainable transmitter or transceiver may be provided in a vehicle for use with remote control systems. A trainable transmitter is configurable by a user to activate one or more of a plurality of different wireless control system receivers using different radio frequency messages. A user may train the trainable transmitter to an existing original transmitter by holding the two transmitters in close range and pressing buttons on the original transmitter and the trainable transmitter. The trainable transmitter identifies the type of remote control system associated with the original transmitter based on a radio frequency signal received from the original transmitter. For example, the trainable transmitter may identify and store the control code and carrier frequency of the original transmitter radio frequency (“RF”) control signal. In addition, the receiver may learn a transmitter identifier of the trainable transmitter. For systems employing a rolling code (or other encryption method), the trainable transceiver and receiver must also be “synchronized” so that the counters of the trainable transmitter and the receiver begin at the same value. Accordingly, the user presses a button on the remote control system receiver to put the receiver in a training mode. A button on the trainable transceiver may then be pressed, for example, two to three times, to transmit messages so the receiver may learn the transmitter identifier, complete synchronization of the receiver and the trainable transmitter and confirm that training was successful. Once trained, the trainable transceiver may be used to transmit RF signals to control the remote control system.
In a transmission mode, a user presses an input device, e.g., a button, of the trainable transmitter that has been trained to a particular remote control system, for example, a garage door opener. In response to the user input, the trainable transmitter retrieves the frequency and control data associated with the button pressed, generates a carrier signal with the appropriate carrier frequency and modulates control data on the carrier signal to generate an RF control signal to control the garage door opener. The RF control signal is then transmitted to the garage door opener. During transmission of an RF signal, however, various RF characteristics of the trainable transmitter circuitry (e.g., RF characteristics of an Application Specific Integrated Circuit (ASIC) used for the RF circuitry) may cause the frequency of the carrier signal to be shifted. For example, RF characteristics may cause a frequency shift during periods when data is modulated on the RF carrier signal. The frequency shift results in a wider bandwidth of the transmitted RF control signal. The receiving bandwidth of a receiver associated with a remote control system, however, may be relatively narrow. Therefore, the wider bandwidth of the RF control signal caused by a shift in carrier frequency may adversely affect performance of the transmitter and the ability of a remote control system receiver to receive and respond to the RF control signal transmitted by the transmitter.
It should be understood that although an electronic transmitter may be described herein with reference to systems for trainable transmitters or rolling code transmitters, one or more of the systems and methods for compensating for frequency shifting may be applied to, and find utility in, other types of transmitters as well. For example, one or more of the systems for compensating for frequency shifting may be suitable for use with fixed code transmitters, single frequency transmitters, etc., all of which may require some form of compensation for frequency shifting.
Accordingly, there is a need for a system and method to compensate for frequency shifts during transmission of an RF control signal. There is also a need for a system and method for compensating for frequency shifts caused when data is being modulated on an RF carrier signal.